A Roman senator from Lucania[4] Severus was one of the last Western emperors, emptied of any effective power (the real power was in the hands of the magister militumRicimer), and unable to solve the many problems affecting the empire; the sources describe him as a pious Christian.[5]

On August 7, 461, the magister militum (commander in chief) of the Western Roman army, Ricimer, had Emperor Majorian killed, thus leaving the western throne empty. A struggle for the succession thus ensued, with the Eastern Emperor, Leo I, the King of the Vandals, Gaiseric, and Ricimer himself involved. The Eastern Emperor traditionally had the right to accept his "colleague," for the empire was nominally still united.

Ricimer needed a weak emperor on the throne, in order to control him: his barbaric descent barred him from taking the throne for himself. Gaiseric had captured the wife and the two daughters of the Western Roman Emperor Valentinian III – Licinia Eudoxia, Placidia and Eudocia – during the sack of Rome (455), and, through the marriage of one of them, Eudocia, with his son Huneric, he had entered the imperial family. Gaiseric's candidate to the Western throne was Olybrius, who had married Placidia and was thus a member of his family.

In order to push for Olybrius' election, Gaiseric decided to put pressure on the empire with several raids on the coasts of Italy and Sicily, maintaining that the peace treaty he had signed with Majorian was no longer valid; Ricimer reacted by sending an embassy to Gaiseric and asking him to respect the treaty, while a second embassy was sent by Leo I asking for the end of the raids and the release of the wife and daughters of Valentinian.

Despite the pressure of the Vandal raids, Ricimer ignored Olybrius and put the senator Libius Severus on the Western throne; he was probably chosen in order to please the Italian aristocracy. Severus was elected Emperor by the Roman Senate on November 19, 461, in Ravenna.[6]

At the beginning of the 460s the Western Roman Empire no longer ruled several imperial provinces even nominally: Britain had been abandoned; Africa had been conquered by the Vandals; and Hispania was occupied by the Suebi and the Visigoths (who were foederati of the empire). However, the area under Libius' control was even smaller, as the governors of several provinces did not recognise him as Emperor: both Aegidius, who controlled Gaul, and Marcellinus, who ruled semi-autonomously over Illyricum, had been supporters of Majorian and thus did not accept Libius' election.

Libius Severus feared that Marcellinus, who commanded a powerful army, could descend upon Italy, and asked for Leo's help; the Eastern Emperor sent Philarcus as envoy to Marcellinus and dissuaded him from the attack. This episode is also important because it marks the passage of Illyricum from the Western to the Eastern sphere of influence.

To oppose Aegidius, Severus appointed his own supporter Agrippinus to the office of magister militum per Gallias, thus officially giving him power over Aegidius. During Majorian's reign Agrippinus had been accused by Aegidius of treachery; found guilty and condemned to death, he had been pardoned, probably because of Ricimer, who then supported him in opposition to Aegidius. Agrippinus asked for support from the Visigoths, and with their help moved against Aegidius and his Frankish allies, led by King Childeric I.

In exchange for their support, in 462 the Visigoths received the city of Narbonne from Severus, thus getting access to the Mediterranean sea and separating Aegidius from the rest of the empire. Among Severus' few official acts is the (464) appointment of Arvandus as Praetorian prefect of Gaul who, in 468, was to be prosecuted for treachery and condemned to death for having tried to obtain the throne.

Therefore, Severus actually ruled only over Italy, even if in 465, with the death of Aegidius, Gaul returned to his sphere of influence for a short time. It is probably to this temporary control over Gaul that the limited issue of his coins by the mint of Arelate is to be dated.

Ricimer put Libius Severus on the throne, though he retained actual power. Some coins exist issued in Severus' name yet bearing a monogram sometimes identified with Ricimer; even if these coins were actually issued in the period between the reign of Severus and of his successor, Anthemius, it is nonetheless an honour unheard of for a barbarian, who was even mentioned on the inscriptions just after the emperors («salvis dd. nn. et patricio Ricimere», CILX, 8072).

Ricimer's control was so clear that, in recording the defeat and death of Bergor, the King of the Alans, by his hand (February 6, 464, near Bergamo), the historian Marcellinus Comes calls Ricimer a king: "Bergor, King of the Alans, is killed by King Ricimer" (Beorgor rex Alanorum a Ricimere rege occiditur, Marcellinus Comes, Chronicle, s.a. 464).

The Vandals continued their raids during the reign of Severus. On one hand Gaiseric justified the raids complaining that he had not received part of Valentinian's legacy; on the other hand, he still hoped to put Olybrius on the Western throne.

Vandal raids deeply affected the economy of the Italian landowners, typically senators; some representatives of the Italian aristocracy went to the Emperor to plead for a reconciliation with Gaiseric. Severus chose the patrician Tatian and sent him to the king of the Vandals, who, however, rejected the peace proposal.

Even if Severus was not officially recognised by Leo I, nonetheless the Eastern and Western Empires did collaborate, as shown by the episode of Leo's intercession with Marcellinus and the embassy led by Philarcus.

Another sign of the collaboration between the two courts is the choice of Consuls. According to tradition, each court chose a consul and accepted the one chosen by the other court. Without Eastern recognition, Severus named himself consul for 462 (his first year as Emperor) and chose an influential member of the Roman senatorial aristocracy (Caecina Decius Basilius, Praetorian prefect of Italy from 463 to 465) for the following year. Severus then decided not to indicate a consul for the years 464 and 465 and chose to accept the two designated by the Eastern court.

The details of Severus' death are obscure, but the majority of modern scholars agree that he died of natural causes in 465. In a passage from his Getica, Jordanes claims Severus ruled for only three years;[8] it is probable, however, that this is a mistake by the 6th-century historian. As regards the day of his death, it is recorded as August 15 by Fasti vindobonenses priores, but a law by Severus dated September 25 has been preserved; either he died after that day or the law was issued after his death in his name.

Cassiodorus, in the 6th century, maintains that Severus was treacherously poisoned by Ricimer in his own palace,[9] but three years after Severus' death, the poet Sidonius Apollinaris wrote that he had died a natural death.[10] According to modern historians, Ricimer had no reason to kill Severus, who was actually a puppet under his control, unless he was an obstacle to Ricimer's reconciliation with Leo.[11]